Apparatus, System, and Method for Mapping the Location of a Nerve

ABSTRACT

An apparatus, system, and method are disclosed for mapping the location of a nerve. The apparatus includes at least one stimulation module, a stimulation detection module, a distance module, and a mapping module. The stimulation module stimulates a nerve with an electrical stimulation current from at least one stimulation electrode. A stimulation detection module detects a muscle reaction resulting from stimulation of the nerve by the at least one stimulation electrode. The distance module uses information from the at least one stimulation electrode and from the stimulation detection module to calculate a distance between the at least one stimulation electrode and the nerve. The mapping module maps a location on the nerve using at least two distances calculated by the distance module and position information of the at least one stimulation electrode for each of the at least two distances calculated.

FIELD OF THE INVENTION

This invention relates to nerve monitoring and more particularly relatesto nerve mapping in three dimensions.

BACKGROUND Description of the Related Art

Back surgery is increasingly done using minimally invasive methods.Nerves which are exposed during open surgical procedures are usually notvisible using a minimally invasive procedure. Nerves which are exposedduring minimally invasive methods are subject to damage during such aprocedure and care should be taken to avoid touching or damaging anerve.

SUMMARY

From the foregoing discussion, it should be apparent that a need existsfor an apparatus, system, and method to map the location of a nerve.Beneficially, such an apparatus, system, and method would map thelocation on the nerve in three dimensions.

The present subject matter has been developed in response to the presentstate of the art, and in particular, in response to the problems andneeds in the art that have not yet been fully solved by currentlyavailable nerve proximity indication systems. Accordingly, the presentsubject matter has been developed to provide an apparatus, system, andmethod for mapping the location of a nerve that overcome many or all ofthe above-discussed shortcomings in the art.

The apparatus to map the location of a nerve is provided with aplurality of modules configured to functionally execute the necessarysteps of stimulating a nerve, detecting a muscle reaction, calculating adistance between a stimulation electrode and a nerve, and mapping thelocation of a nerve using at least two distance calculations. Thesemodules in the described embodiments include at least one stimulationelectrode, a stimulation detection module, a distance module, and amapping module.

In certain embodiments the at least one stimulation electrode stimulatesa nerve with an electrical stimulation current from an electricalsource. In one embodiment a stimulation detection module detects amuscle reaction resulting from stimulation of the nerve by the at leastone stimulation electrode. The distance module may use information fromthe at least one stimulation electrode and from the stimulationdetection module to calculate a distance between the at least onestimulation electrode and the nerve. The mapping module maps a locationon the nerve using at least two distances calculated by the distancemodule and position information of the at least one stimulationelectrode for each of the at least two distances calculated. In oneembodiment the map of the location on the nerve indicates a location ofa point on the nerve.

In certain embodiments the stimulation detection module includes anelectromyograph that detects an electrical potential generated by amuscle cell in response to stimulation of the nerve by the at least onestimulation electrode.

In one embodiment the at least two distances calculated by the distancemodule includes at least three distances. In certain embodiments eachdistance calculated indicates a spherical locus of potential sites onthe nerve equidistant from a position of the at least one stimulationelectrode when the at least one stimulation electrode stimulates thenerve. In one embodiment the mapping module maps the location on thenerve by determining an intersection of the spherical locus of potentialsites.

The at least one stimulation electrode, in one embodiment, includes afirst stimulation electrode, a second stimulation electrode, and a thirdstimulation electrode. In certain embodiments the first stimulationelectrode stimulates the nerve from a first position to calculate afirst distance. The second stimulation electrode, in certainembodiments, stimulates the nerve from a second position to calculate asecond distance. The third stimulation electrode stimulates the nervefrom a third position to calculate a third distance. In one embodimentthe mapping module maps a first location on the nerve using the firstdistance, the second distance, and the third distance. In oneembodiment, the calculated distance may be elliptical or some othernon-spherical shape based on the electrical field associated with eachelectrode.

The apparatus, in certain embodiments, includes an electrode positioningmodule that moves at least one of the first stimulation electrode, thesecond stimulation electrode, and the third stimulation electrode to anew position. In one embodiment, at each new position of a stimulationelectrode, the stimulation detection module, the distance module, andthe mapping module determine one or more additional locations on thenerve. In certain embodiments the mapping module maps a route of thenerve using the one or more additional locations on the nerve.

In a further embodiment, the electrode positioning module moves thefirst stimulation electrode, the second stimulation electrode, and thethird stimulation electrode to obtain additional distance calculationsand map one or more additional locations on the nerve.

The at least one stimulation electrode, in one embodiment, includes asingle stimulation electrode and also includes an electrode positioningmodule. The electrode positioning module positions the stimulationelectrode in at least three positions to calculate at least threedistances using the stimulation electrode and the distance module.

In certain embodiments, the electrode positioning module moves thestimulation electrode to a new position to determine one or moreadditional locations on the nerve using the stimulation electrode, thestimulation detection module, the distance module, and the mappingmodule. In one embodiment the mapping module maps a route of the nerveusing the one or more additional locations on the nerve.

The apparatus, in certain embodiments, also includes an imaging moduleand an overlay module. The imaging module captures an image of apatient's anatomy. The overlay module, in one embodiment, overlays a mapof the location of the nerve on the image of the patient's anatomy. Incertain embodiments the image is captured by the imaging module and themap is mapped by the mapping module.

In a further embodiment the apparatus also includes a marking module.The marking module, in one embodiment, marks a position of the at leastone stimulation electrode with a marker. The marker may be made of amaterial detectable by the imaging module. The overlay module, in oneembodiment, uses the marker to position the map of the location on thenerve on the image of the patient's anatomy captured by the imagingmodule.

In certain embodiments the imaging module captures a three dimensionalimage of the patient's anatomy. In one embodiment the overlay moduleoverlays the map of the location on the nerve in three dimensions suchthat the location on the nerve within the patient's anatomy isidentified in three dimensions. The imaging module, in certainembodiments, includes one or more of an imaging device selected from anx-ray device, a computerized axial tomography device, a magneticresonance imaging device, and an ultrasound device.

A method of the present subject matter is also presented for mapping thelocation of a nerve. The method in the disclosed embodimentssubstantially includes the steps necessary to carry out the functionspresented above with respect to the operation of the described apparatusand system. In one embodiment, the method includes stimulating a nervewith a stimulation current from an electrical source using at least onestimulation electrode.

The method also may include detecting a muscle reaction resulting fromstimulation of the nerve by the at least one stimulation electrode. Incertain embodiments a distance between the at least one stimulationelectrode and the nerve is calculated using current information from theat least one stimulation electrode at a time of first detecting themuscle reaction. The method includes other methods of detecting thenerve stimulation using a nerve response, a spinal cord response, or asomato-sensory response. The method also includes stimulating one ormore peripheral nerves distal to the spinal cord and using the multipleelectrodes as pickup electrodes for the resulting nerve activity.

The method may also include mapping a location on the nerve using atleast two distances calculated and position information of the at leastone stimulation electrode for each of the at least two distancescalculated. In a further embodiment, the mapping of the location on thenerve includes determining an intersection of a spherical locus ofpotential sites of the nerve equidistant from a position of the at leastone stimulation electrode when the at least one stimulation electrodestimulates the nerve.

In one embodiment, the at least two distances calculated includes atleast three distances. Each of the at least three distances calculatedindicates a spherical locus of potential sites of the nerve equidistantfrom a position of the at least one stimulation electrode when the atleast one stimulation electrode stimulates the nerve. In one embodimentthe location on the nerve is determined by determining an intersectionbetween the spherical locus of potential sites indicated by the at leastthree distances.

In a further embodiment an image of the patient's anatomy may becaptured and a map of the location of the nerve may be overlaid on theimage of the patient's anatomy.

Reference throughout this specification to features, advantages, orsimilar language does not imply that all of the features and advantagesthat may be realized with the subject matter should be or are in anysingle embodiment. Rather, language referring to the features andadvantages is understood to mean that a specific feature, advantage, orcharacteristic described in connection with an embodiment is included inat least one embodiment. Thus, discussion of the features andadvantages, and similar language, throughout this specification may, butdo not necessarily, refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics ofthe subject matter may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize that thesubject matter may be practiced without one or more of the specificfeatures or advantages of a particular embodiment. In other instances,additional features and advantages may be recognized in certainembodiments that may not be present in all embodiments.

These features and advantages of the present subject matter will becomemore fully apparent from the following description and appended claims,or may be learned by the practice of the subject matter as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the advantages will be readily understood, a moreparticular description of the subject matter briefly described abovewill be rendered by reference to specific embodiments that areillustrated in the appended drawings. Understanding that these drawingsdepict only typical embodiments and are not therefore to be consideredto be limiting of its scope, the subject matter will be described andexplained with additional specificity and detail through the use of theaccompanying drawings, in which:

FIG. 1 depicts a side view of one embodiment of a system for mapping thelocation of a nerve using a single stimulation electrode;

FIG. 2 depicts a side view further illustrating one embodiment of thesystem for mapping the location of a nerve using a single stimulationelectrode of FIG. 1 with the stimulation electrode repositioned to asecond position;

FIG. 3 depicts a side view of one embodiment of a system for mapping thelocation of a nerve using multiple stimulation electrodes;

FIG. 4 depicts a block diagram of one embodiment of an apparatus 400 tomap the location of a nerve;

FIG. 5 depicts a side view further illustrating one embodiment of theelectrode positioning module of FIG. 4;

FIG. 6A depicts one embodiment of an enlarged cross sectional view of atip area of a stimulation electrode;

FIG. 6B depicts one embodiment of an enlarged cross sectional view of alead coupling area of a stimulation electrode;

FIG. 7 depicts one embodiment of a portion of a patient's spinal columnwith the positioning module positioning three stimulation electrodesnear the patient's spinal column and an imaging module having twoimaging devices; and

FIG. 8 depicts a schematic block diagram of one embodiment of a methodfor mapping the location of a nerve; and

FIG. 9 is a schematic block diagram depicting another embodiment of amethod for mapping the location of a nerve.

DETAILED DESCRIPTION

Reference throughout this specification to “one embodiment,” “anembodiment.” or similar language means that a particular feature,structure, or characteristic described in connection with the embodimentis included in at least one embodiment. Thus, appearances of the phrases“in one embodiment,” “in an embodiment,” and similar language throughoutthis specification may, but do not necessarily, all refer to the sameembodiment.

Furthermore, the described features, structures, or characteristics ofthe subject matter may be combined in any suitable manner in one or moreembodiments. In the following description, numerous specific details areprovided, such as examples of programming, software modules, userselections, network transactions, database queries, database structures,hardware modules, hardware circuits, hardware chips, etc., to provide athorough understanding of embodiments of the subject matter. One skilledin the relevant art will recognize, however, that the subject matter maybe practiced without one or more of the specific details, or with othermethods, components, materials, and so forth. In other instances,well-known structures, materials, or operations are not shown ordescribed in detail to avoid obscuring aspects of the subject matter.

The schematic flow chart diagrams included herein are generally setforth as logical flow chart diagrams. As such, the depicted order andlabeled steps are indicative of one embodiment of the presented method.Other steps and methods may be conceived that are equivalent infunction, logic, or effect to one or more steps, or portions thereof, ofthe illustrated method. Additionally, the format and symbols employedare provided to explain the logical steps of the method and areunderstood not to limit the scope of the method. Although various arrowtypes and line types may be employed in the flow chart diagrams, theyare understood not to limit the scope of the corresponding method.Indeed, some arrows or other connectors may be used to indicate only thelogical flow of the method. For instance, an arrow may indicate awaiting or monitoring period of unspecified duration between enumeratedsteps of the depicted method. Additionally, the order in which aparticular method occurs may or may not strictly adhere to the order ofthe corresponding steps shown.

One of skill in the art will recognize that an electrical currentapplied to a nerve initiates a muscle contraction. Further, one of skillin the art will recognize that during voluntary as well as involuntarycontraction a muscle cell creates an observable electrical potential. Bystimulating a nerve with an electrical current from a stimulationelectrode the muscle cells that the stimulated nerve innervates willcreate an electrical potential. Nerves are sensitive to electricalstimulation and the amount of charge needed to depolarize a nerve can beused to estimate the distance of the nerve from the stimulatingelectrode. Nerve depolarization may be measured indirectly by detectingelectromyogram activity in a muscle associated with the nerve.

Dilators and other surgical instruments which are inserted and used tocut or stretch tissue may be equipped with stimulating sites to inducean electromyogram response in a muscle associated with a nerve. Anelectromyogram response in a muscle associated with a nerve may indicatenerve proximity but the practitioner is left to guess as to the locationof the nerve.

The subject matter of the present disclosure is directed to a nervemapping system, method and apparatus that generates a virtual threedimensional view of a nerve or nerves. The three dimensional view can beused to guide a dilator or other surgical instrument past vulnerablenerve roots.

Nerve depolarization is an ‘all or none’ phenomenon. A peripheral nervecontains hundreds to thousands of neurons, each of which fires tens tothousands of muscle fibers. Electrical depolarization occurs abruptlywhen enough total charge is forced across an individual neuron, and eachneuron that is depolarized adds to an electromyogram response.

Using established models for nerve depolarization, and establishedmodels for electrical fields in volume conductors, the present subjectmatter allows the computation of a distance from a stimulus site to anerve as a function of other variables that can be either controlled orestimated.

The primary nerve characteristic of interest is its depolarizationcharge. A nerve's depolarization charge varies across individual neuronsbut has a statistically well-defined distribution. The minimal chargefor the first few neurons to depolarize is well-defined and repeatable.

A charge delivered to a nerve from a stimulation electrode is a functionof several variables, including (1) the stimulator output level, (2) thestimulus duration and polarity, (3) the absolute distance from thestimulation electrode, and (4) the orientation of the field in the senseof a three axis vector.

FIG. 1 depicts one embodiment of a system 100 for mapping the locationof a nerve 104 using a single stimulation electrode 102 positioned at afirst position 103. With a stimulation electrode 102 positioned at afirst position 103, the distance 112 between a tip 106 of thestimulation electrode 102 and the nerve 104 can be measured by adjustingthe stimulation current provided to the tip 106 of the stimulationelectrode 102 until a threshold electromyogram response is detected in amuscle located some distance away from the stimulation site. In certainembodiments the stimulation current may be adjusted or controlledthrough the use of a second stimulation electrode (not shown), anindifferent electrode (not shown), or in combination.

In certain embodiments the muscle or muscles exhibiting the thresholdelectromyogram response may give a skilled practitioner additionalinsight into which of several nerves 104 is being stimulated. In certainembodiments knowledge of human anatomy will give one of ordinary skillin the art insight into the route a particular nerve 104 typically takeswithin a human body.

Typically, the larger the distance 112 between the stimulation electrode102 and the nerve 104 the higher the stimulation current required toelicit an electromyogram response in the muscle. In one embodimentCoulombs law, also known as the inverse problem for source localization,may be used to determine the distance 112 between the stimulationelectrode 102 and the nerve 104. In other embodiments other formula'smay be used to determine the distance 112 between the stimulationelectrode 102 and the nerve 104 as a function of a stimulation currentrequired to invoke an electromyogram response in the muscle.

The applicable form of Coulombs law can be expressed mathematically asQ=k(Q₀/r²) where Q is the required stimulating charge. K is a functionof the nerve, Q₀ is the minimum charge needed to stimulate the nerve104, and r is the distance 112 between the stimulation electrode 102 andthe nerve 104 (represented in FIG. 1 as the distance between the tips ofthe arrow 112). Therefore, one of skill in the art will recognize thatwhere the required stimulating charge E and the minimal charge needed tostimulate the nerve 104 are known, the distance r (112) between thestimulation electrode 102 and the nerve 104 can be calculated. Thestimulating current and charge are related by the stimulus durationwhich is generally fixed. Use of the term “stimulating current” is usedhereafter.

One of skill in the art will recognize that variables such as animpedance level of the tissue surrounding the tip 106 of the stimulationelectrode 102, the nerve geometry, the stimulation electrode 102characteristics, stimulation electrode 102 geometry, whether or not thestimulation electrode 102 is used as the anode or cathode, etc. mayaffect the accuracy of the distance calculation. One of ordinary skillof one in the art may account for these external variables in making thedistance calculations.

In certain embodiments a single stimulation electrode 102 used todetermine the distance 112 between the stimulation electrode 102 and thenerve 104 gives an absolute distance using Coulomb's law discussedabove. While the distance 112 between the stimulation electrode 102 andthe nerve 104 may be determined, the calculation may typically beinterpreted as a spherical locus of potential sites 108 of the nerve 104equidistant from a position (in this case the first position 103) whenthe stimulation electrode 102 stimulates the nerve 104. The direction ofthe nerve 104 from the stimulation site at the tip 106 of thestimulation electrode 102 is typically unknown.

After a single distance has been determined, the calculation indicatesthat the nerve 104 may be located anywhere on the spherical locus ofpotential sites 108. Thus, while stimulation current may actuallystimulate the nerve 104 at the interface 110 between the stimulationcurrent and the nerve 104, the distance calculation typically onlyindicates that the nerve 104 is a certain distance away from thestimulation site which may be any point on the spherical locus ofpotential sites 108.

FIG. 2 depicts one embodiment of a system 200 for mapping the locationof a nerve 104 using a single stimulation electrode 102 which ispositioned at a first position 103 and then repositioned to a secondposition 201. In the embodiment illustrated in FIG. 2, the repositionedstimulation electrode 202 is illustrated in a dashed outline indicatingthat the stimulation electrode 102 has been repositioned to the secondposition 201.

Using the repositioned stimulation electrode 202 positioned at thesecond position 201, the distance 212 between the tip 206 of therepositioned stimulation electrode 202 and the nerve 104 can be measuredby adjusting the stimulation current provided to the tip 206 of therepositioned stimulation electrode 202 until a threshold electromyogramresponse is detected in the muscle located some distance away from thestimulation site.

The stimulation current I required to invoke the thresholdelectromyogram response with the repositioned stimulation electrode 202positioned in the second position 201 can be used to calculate adistance 212 between the tip 206 of the repositioned stimulationelectrode 202 and the nerve 104 using Coulomb's law discussed above.Again, the calculation of the distance 212 between the tip 206 of therepositioned stimulation electrode 202 and the nerve 104 may beinterpreted as a spherical locus of potential sites 208 with each siteequidistant from the stimulation site at the tip 206 of the repositionedstimulation electrode 202. Thus, the nerve 104 may be located at anyposition on the spherical locus of potential sites 208.

If the first position 103 of the tip 106 of the stimulation electrode102 and the second position 201 of the tip 206 of the repositionedstimulation electrode 202 are known, an estimation of the actuallocation of the nerve 104 can be narrowed to a position fallingsomewhere on the intersection of spherical locus of potential sites 108and spherical locus of potential sites 208 which is a circle 214. Thus,in certain embodiments, where a single stimulation electrode 102 is usedto determine two distances, the position of the nerve 104 may typicallybe pinpointed with an accuracy of circle 214.

While the embodiment illustrated in FIG. 2 depicts the stimulationelectrode 102 as being repositioned along an axis of the stimulationelectrode 102, one of skill in the art will recognize that thestimulation electrode 102 may also be repositioned in a directiontransverse to the axis of the stimulation electrode 102 to obtaindistance calculations between the tip 106 of the stimulation electrode102 and the nerve 104. Using a single stimulation electrode 102 in thismanner, the user is generating a mental image of the nerve 104 byprobing forward and back and side to side and turning the stimulationcurrent up and down to determine a distance between the tip 106 of thestimulation electrode 102 and the nerve 104 at various locations.

In certain embodiments the two distances calculated, the first being thedistance calculated between the tip 106 of the stimulation electrode 102and the nerve 104 with the stimulation electrode 102 positioned at thefirst position 103, and the second being the distance calculated betweenthe tip 206 of the repositioned stimulation electrode 202 and the nerve104 with the repositioned stimulation electrode 202 positioned at thesecond position 201, may be enough to determine a position of the nerve104. For example, in highly uniform nerves 104, that is, with nerves 104that do not typically vary significantly in location from patient topatient, only one location on the circle 214 or a section of the circle214 may make sense to one of skill in the art and other locations on thecircle 214 may be ruled out as a possible location of the nerve 104. Asection of the circle 214, in one embodiment, may be accurate enough forthe purposes of a procedure. In one embodiment a precise calculation ofa location of a nerve 104 may not be necessary. In such an embodimentknowing that the nerve 104 lies somewhere on the circle 214 may beenough to perform certain medical procedures.

In other embodiments additional distance calculations may be performedwith the stimulation electrode 102 positioned at three or more positionswithin a patient. A third distance calculation, with the stimulationelectrode 102 positioned at a third position, may map the position onthe nerve 104 to two points on circle 214 as further discussed withreference to FIG. 3.

Typically a nerve 104 is a chord-like structure that may run throughouta patient's body to control various muscle functions in the patient.Therefore, one of skill in the art will recognize that in certainembodiments the phrase “position on the nerve” may be used to indicate apoint on the nerve 104 and not necessarily the entire chord-likestructure of the nerve 104. As discussed below, in certain embodimentsadditional points or positions on the nerve 104 may be determined usingthe methods, apparatuses and systems described herein to map a path orroute of the nerve 104 as it passes through the patient's anatomy.

While the embodiments discussed thus far have been described withreference to the stimulation electrode 102 positioned within a patient,one of skill in the art will recognize that in certain embodiments thestimulation electrode 102 may be a surface electrode positioned on asurface of the patient's skin.

FIG. 3 depicts one embodiment of a system 300 for mapping the locationof a nerve 104 using three stimulation electrodes 302 which arepositioned at three positions 303. While the embodiment illustrated inFIG. 3 depicts three separate stimulation electrodes 302, one of skillin the art will recognize that in certain embodiments a single electrode302 may be used and repositioned to three different positions 303. Inthe embodiment illustrated in FIG. 3, the nerve 104 has been omitted forclarity.

In certain embodiments the stimulation electrodes 302 may be positionedon or within a patient at three separate positions 303. Each stimulationelectrode 302 may then deli ver a stimulation current I to the patient.In certain embodiments the stimulation current I may be increased untila threshold electromyogram response is observed in a muscle. In oneembodiment the stimulation current I is delivered to each stimulationelectrode 302 one at a time so that a practitioner can determine whichof the three stimulation electrodes 302 is evoking the thresholdelectromyogram response. In other embodiments the stimulation currentmay be delivered to two or more of the stimulation electrodes 302 at thesame time.

In certain embodiments, once the stimulation current required to invokethe threshold electromyogram response in the muscle for each stimulationelectrode 302 is determined, Coulomb's law may be used to calculate thedistance between each stimulation electrode 302 and the nerve 104 (FIGS.1 and 2). Each distance calculated may be interpreted as a sphericallocus of potential sites 308 of the nerve 104 equidistant from aposition 303 of the stimulation electrode 302 when the stimulationelectrode 302 stimulates the nerve 104.

As discussed above, if the position 303 of the stimulation electrodes302 are known, an estimation of the actual location of the nerve 104 maybe narrowed to a position falling somewhere on an intersection of two ofthe spherical locus of potential sites (in this case the intersection ofspherical locus of potential sites 308 a and 308 b) which is a circle314. An intersection of the circle 314 and a third spherical locus ofpotential sites 308 c may be used to pinpoint the location of the nerve104 to two positions 316 on the circle 314 as defined by theintersection of spherical locus of potential sites 308 a and 308 b.

In certain embodiments pinpointing the location of the nerve 104 to oneor two positions 316 on the circle 314 defined by the intersection ofspherical locus of potential sites 308 a and 308 b may give apractitioner sufficient resolution to perform a surgical or othermedical procedure. In other embodiments additional distance calculationsmay be performed, either with additional stimulation electrodes 302 orby repositioning one of the stimulation electrodes 302, to give thepractitioner greater insight into the position of the nerve 104.

FIG. 4 depicts a block diagram of one embodiment of an apparatus 400 tomap the location of a nerve such as nerve 104 of FIGS. 1 and 2. Theapparatus 400 may include at least one stimulation electrode 102, atleast one electromyogram pickup electrode 416, and a control module 401.In one embodiment the control module 401 includes a stimulation module402, a stimulation detection module 404, a distance module 406, and amapping module 408. In certain embodiments the control module 401 mayalso include an electrode positioning module 410, a position detectionmodule 412, an imaging module 414, an overlay module 418, and a markingmodule 420.

In certain embodiments the stimulation module 402 stimulates a nerve 104with an electrical stimulation current from at least one stimulationelectrode 102. In one embodiment the stimulation current is provided byan electrical source 403. In certain embodiments the electrical source403 may be internal to the apparatus 400, that is, in one embodiment theapparatus 400 may include an internal electrical source 403 such as abattery. In other embodiments the electrical source 403 may be externalto the apparatus 400.

In one embodiment the stimulation electrode(s) 102 may be directlyelectrically coupled to the electrical source 403. In such an embodimentthe stimulation electrode(s) 102 may provide feedback to the controlmodule 401 for use in determining the location of a nerve 104. Incertain embodiments feedback from the stimulation electrode(s) 102 mayinclude one or more of position information identifying a position ofthe stimulation electrode(s) 102 when the stimulation electrode(s) 102is stimulating a nerve 104, information about the amplitude of thestimulation current delivered to the stimulation electrode(s) 102,information about the frequency of the stimulation current delivered tothe stimulation electrode(s) 102, information regarding the timing ofthe stimulation current delivered to the stimulation electrode(s) 102,etc.

In other embodiments the electrical source 403 may be coupled to thecontrol module 401 and the control module 401 may provide an electricalcurrent from the electrical source 403 to the stimulation electrode(s)102. In such an embodiment the control module 101 may control theposition of the stimulation electrode(s) 102 with an electrodepositioning module 410, the amplitude of the stimulation currentdelivered to the stimulation electrode(s) 102, the frequency of thestimulation current delivered to the stimulation electrode(s) 102,and/or information regarding the timing of the stimulation currentdelivered to the stimulation electrode(s) 102. Thus, in certainembodiments feedback from the stimulation electrode(s) 102 may beunnecessary as the control module 401 may already have such informationby virtue of the control module 401 controlling the amplitude,frequency, and timing of the stimulation current as well controlling theposition of the stimulation electrode(s) 102.

The stimulation detection module 404 detects a muscle reaction. In oneembodiment the muscle reaction includes a response in a muscle resultingfrom stimulation of the nerve by the at least one stimulation electrode102.

In certain embodiments the stimulation detection module 404 may includean electromyograph coupled to an electromyogram pickup electrode 416,directly, through the control module 401, through another module, or thelike. The electromyogram pickup electrode 416 detects an electricpotential generated by a muscle cell or group of muscle cells inresponse to stimulation of the nerve 104 by the at least one stimulationelectrode 102. In certain embodiments the electromyogram pickupelectrode 416 may be a surface electrode or electrodes. In otherembodiments the electromyogram pickup electrode 416 may be anintramuscular electrode or electrodes positioned within the muscletissue of the patient.

In one embodiment a physician, technician or other user may visuallyobserve a muscle response caused by the stimulation electrode 102stimulating the nerve 104 with the stimulation current. In such anembodiment the electromyograph may be unnecessary and may be omitted ormay supplement physical observations.

The distance module 406, in one embodiment, uses information from thestimulation module 402 and from the stimulation detection module 404 tocalculate a distance 112 between the stimulation electrode 102 and thenerve 104. In certain embodiments the information from the stimulationmodule 402 includes information about the threshold current that causesa muscle reaction. The information from the stimulation detection module404 may include information about which muscle reacted to stimulation ofthe nerve 104 by the stimulation electrode 102. In one embodiment theinformation from the stimulation detection module 404 may simply includeinformation indicating that a muscle responded, regardless of whichmuscle responded.

For example, in certain embodiments the stimulation electrode 102 ispositioned on or within a patient. A stimulation current is providedthrough the stimulation electrode 102 and is increased until thestimulation detection module 404 detects a threshold response in amuscle, either through visual observation or by the electromyograph.Once a threshold response is observed, the stimulation current requiredto invoke the threshold response in the muscle may be used to calculatea distance 112 between the stimulation electrode 102 and the nerve 104.

In one embodiment the distance module 406 may use Coulomb's equationdiscussed above to calculate the distance 112 between the stimulationelectrode 102 and the nerve 104. In other embodiments other formula'smay be used to determine the distance 112 between the stimulationelectrode 102 and the nerve 104 as a function of a stimulation currentrequired to invoke an electromyogram response in the muscle.

The mapping module 408, in one embodiment, maps a location on the nerve104 using at least three of the distances calculated by the distancemodule 406. In certain embodiments the mapping module 408 also usesposition information of the stimulation electrode 102 to map thelocation on the nerve 104.

For example, referring again to FIG. 3, in certain embodiments thedistance module 406 calculates a series of distances for each position303 of the stimulation electrodes 302. The distances calculated indicatea spherical locus of potential sites 308 of the nerve 104 equidistantfrom a position 303 of the stimulation electrode 302 when thestimulation electrode 302 stimulates the nerve 104.

In the embodiment illustrated in FIG. 3 there are three stimulationelectrodes 302, a first stimulation electrode 302 a, a secondstimulation electrode 302 b, and a third stimulation electrode 302 c.The first stimulation electrode 302 a stimulates the nerve 104 from afirst position 303 a to calculate a first distance. The secondstimulation electrode 302 b stimulates the nerve 104 from a secondposition 303 b to calculate a second distance. The third stimulationelectrode 302 c stimulates the nerve 104 from a third position 303 c tocalculate a third distance.

The mapping module 408 maps a first location on the nerve 104 using thefirst distance, the second distance, and the third distance. In certainembodiments the mapping module 408 uses a known position of eachstimulation electrode 302 (or a single stimulation electrode 102) todetermine where the spherical loci of potential sites 308 overlap.

In certain embodiments the position detection module 412 may know theposition of each stimulation electrode 302 (or a single stimulationelectrode 102) by virtue of an electrode positioning module 410controlling the position of each stimulation electrode 302 (or a singlestimulation electrode 102).

In other embodiments the position of each stimulation electrode 302 (ora single stimulation electrode 102) may be known to the positiondetection module 412 by virtue of feedback information provided by thestimulation electrodes 302 (or the single stimulation electrode 102) tothe position detection module 412. One of skill in the art willrecognize that the position information of the stimulation electrodes302 (or the single stimulation electrode 102) may only be relevant wherethe position information relates to the position of the stimulationelectrodes 302 (or the single stimulation electrode 102) when thestimulation electrodes 302 (or the single stimulation electrode 102) isstimulating the nerve 104.

As discussed above, the intersection of three spherical loci ofpotential sites 308 may result in two potential two positions 316 on thecircle 314 where the nerve 104 may be located. In other embodiments themapping module 408 may use more than three distance calculations to moreprecisely identify the location on the nerve 104.

In certain embodiments the apparatus 400 may include an electrodepositioning module 410. The electrode positioning module 410 mayposition the stimulation electrodes 302 (or the single stimulationelectrode 102) in position to calculate a first set of distances betweenthe tip 306 of each stimulation electrode 302 (or the tip 106 of thesingle stimulation electrode) and the nerve 104. These distances may beused by the mapping module 408 to map the location on the nerve 104.

In one embodiment the electrode positioning module 410 moves at leastone of the first stimulation electrode 302 a, the second stimulationelectrode 302 b, and the third stimulation electrode 302 c to a newposition. At each new position of a stimulation electrode 302, thestimulation detection module 404, the distance module 406, and themapping module 408 may be used to determine one or more additionallocations on the nerve 104. In certain embodiments the additionallocations may be determined in a manner substantially similar to themanner in which the first location on the nerve 104 is determined.

In one embodiment the electrode positioning module 410 moves the firststimulation electrode 302 a, the second stimulation electrode 302 b, andthe third stimulation electrode 302 c. The stimulation electrodes 302may be moved individually or in unison. At each new position thedistance module 406 may calculate a new distance in a mannersubstantially similar to the manner in which the distance module 406calculates the original distances. The mapping module 408 may then usenew distance, along with two other distances, either the originaldistances or new distance calculated for each stimulation electrode 302,to map an additional location on the nerve 104.

In another embodiment, such as where the apparatus 400 includes a singlestimulation electrode 102, the electrode positioning module 410 mayposition the stimulation electrode 102 in three positions to calculatethree distances using the stimulation electrode 102 and the distancemodule 406. In certain embodiments the electrode positioning module 410may move the stimulation electrode 102 to fourth position to determineone or more additional locations on the nerve 104 using the stimulationelectrode 102, the distance module 406, and the mapping module 408. Inthis manner, every time the stimulation electrode 102 is moved, a newdistance may be calculated which can be used by the mapping module 408to map a new location on the nerve 104.

In one embodiment the mapping module 408 may map a route of the nerve104 as it passes through a patient's body using one or more additionallocations on the nerve 104. For example, in certain embodiments a firstlocation on the nerve 104 may first be mapped by the mapping module 408using three distances calculated by the distance module 406 as outlinedabove. One or more of the stimulation electrodes 302 (or the singlestimulation electrode 102) may then be repositioned by the electrodepositioning module 410 and a new distance may be calculated by thedistance module 406. In other embodiments the electrode positioningmodule 410 may reposition all three stimulation electrodes 302 to getthree new distance calculations. In yet another embodiment the electrodepositioning module 410 may reposition a single stimulation electrode 102at an additional three positions to calculate three additionaldistances.

In certain embodiments the mapping module 408 may then use the newdistance calculation along with two of the original distancecalculations to map an additional location on the nerve 104. In otherembodiments the mapping module 408 may use the three new distancecalculations to map an additional location on the nerve 104.

A line connecting the first location on the nerve 104 and the additionallocation on the nerve 104 may indicate a route of the nerve 104 as itpasses between the first location and the additional location. Incertain embodiments mathematical procedures for finding the best fittingcurve, such as least squares fitting between the first location and thesecond location, may be used to determine the route of the nerve 104.

While the embodiments discussed with relation to apparatus 400 includethree distance calculations, one of skill in the art will recognize thatin certain embodiment's one or two distance calculations may give apractitioner enough information to locate a nerve 104. Thus, in certainembodiments a single distance or two distance calculations may be usedto map the position of the nerve 104 where resolution is not asimportant. In other embodiments more than three distance calculationsmay be used to map the location of a nerve 104.

In one embodiment the apparatus 400 may include an imaging module 414that captures an image of a patient's anatomy. In certain embodimentsthe imaging module 414 may include one or more of an x-my device, acomputerized axial tomography device, a magnetic resonance imagingdevice, and an ultrasound device or any other device that captures animage of a patient's anatomy which is known in the art. In certainembodiments the imaging module 414 may capture a three dimensional imageof the patient's anatomy by capturing an image of the patient's anatomyfrom at least two angles.

In certain embodiments an overlay module 418 overlays a map of thelocation of the nerve 104 mapped by the mapping module 408 on the imageof the patient's anatomy captured by the imaging module 414. Where theimage captured by the imaging module 414 is a three dimensional image,the overlay module 418 may overlay the map of the location on the nerve104 using x, y, and z coordinates of a Cartesian coordinate system suchthat the location on the nerve 104 within a patient's anatomy isidentified in three dimensions. With the nerve 104 mapped in threedimensional space a physician or other practitioner can safely avoiddamaging the nerve 104 during a surgical or other medical procedure.

In one embodiment a marking module 420 marks a position of thestimulation electrodes 302 (or the single stimulation electrode 102)with a marker. In certain embodiments the marker is made of a materialdetectable by the imaging module 414. The overlay module 418 may use themarker to position the map of the location on the nerve 104 on the imageof the patient's anatomy captured by the imaging module 414. In certainembodiments the stimulation electrodes 302 (or the single stimulationelectrode 102) may be made of a material detectable by the imagingmodule 414. In other embodiments a separate marker made of a materialdetectable by the imaging module 414 may be used to mark the position ofthe stimulation electrodes 302 (or the single stimulation electrode102).

FIG. 5 depicts one embodiment of the electrode positioning module 410 ofthe system 400 of FIG. 4. In certain embodiments the electrodepositioning module 410 includes a stimulation electrode mounting member502, a rigid mount 506, and a slideable coupling 508.

In one embodiment the stimulation electrodes 302 extend through and aremounted to the electrode mounting member 502. The stimulation electrodes302, in certain embodiments, are rigidly mounted to the electrodemounting member 502. In other embodiments the stimulation electrodes 302may be slideably mounted to the electrode mounting member 502 such thateach stimulation electrode 302 may be individually positioned to adeeper or shallower position within a patient's anatomy.

In embodiments where the stimulation electrodes 302 are rigidly mountedto the electrode mounting member 502 the electrode positioning module410 may operate to control the depth of the electrodes within apatient's anatomy. The electrode mounting member 502 may be coupled tothe slideable coupling 508. In one embodiment, the slideable coupling508 slides along rigid mount 506 to position or reposition thestimulation electrodes 302 within the patient's anatomy.

In certain embodiments the electrode positioning module 410 may includea manual adjusting member (not shown) that allows a physician or otheruser to manually adjust a depth of the stimulation electrodes 302. Insuch and embodiment the slideable coupling 508 may include depthindicators 510 which correspond with depth indicators 512 located on therigid mount 506. The depth indicators 510, 512 may be used to give aphysician or other operator and indication of the depth of thestimulation electrodes 302 within the patient's anatomy.

In other embodiments the electrode positioning module 410 may be coupledto the control module 401 of apparatus 400 and the control module 401may control the depth of the stimulation electrodes 302. In such anembodiment the electrode positioning module 410 may include a mechanicaladjusting member (not shown) that adjusts the depth of the stimulationelectrodes 302. One of skill in the art will recognize that themechanical adjusting member (not shown) may be incorporated into theelectrode positioning module 410 in a variety of differentconfigurations.

In certain embodiments the electrode positioning module 410 allows theuser to advance the electrodes 302 into a patient's anatomy in astepwise fashion. Thus, the electrode positioning module 410 may firstposition the stimulation electrodes 302 in the patient. The thresholdstimulation current required to invoke an electromyogram response maythen be determined for each stimulation electrode 302 and the distancemodule 406 may convert the threshold stimulation current data intodistances as described above. The mapping module 408 may use thedistance calculations, along with the position information of eachstimulation electrode 302 to map a location on the nerve 104. In certainembodiments the electrode positioning module 410 may then reposition thestimulation electrodes 302 and the process just described may berepeated to map an additional location on the nerve 104.

In certain embodiments the electrode positioning module 410 may assistthe position detection module 412 in determining the position of thestimulation electrodes 302 when the stimulation electrodes 302 arepositioned within a patient. For example, if the distance between theelectrode mounting member 502 and the tip 306 of a particularstimulation electrode 302 is known, and the angle at which thestimulation electrode 302 is inserted into a patient's anatomy alsoknown, the location of the tip 306 of the stimulation electrode 302relative to the electrode mounting member 502 can be determined.

In one embodiment the electrode mounting member 502 includes a number ofguide holes 514 for guiding a physician or other user in positioning adilator, needle, or other surgical or medical instrument within apatient's anatomy. In certain embodiments, once the location of a nerve104 or the route of the nerve 104 has been mapped, the stimulationelectrodes 302 may remain positioned within the patient. The physicianor other user may use the stimulation electrodes 302 as a reference ofwhere to position the surgical or other medical instrument and mayselect a guide hole 514 that will guide the surgical or other medicalinstrument into the patient's anatomy without interfering with ordamaging the nerve 104. In certain embodiments the guide holes 514 maybe replaced with a multi-axis aiming system that guides a physiciansplacement of the surgical or other medical instrument.

In the embodiment illustrated in FIG. 5 three stimulation electrodes 302are mounted to the substantially rigid mounting member 504 in atriangular pattern. In other embodiments more than three stimulationelectrodes 302 may be mounted to the substantially rigid mounting member504 in a variety of patterns to improve resolution of the apparatus 400.In certain embodiments two electrodes 302 inserted with the correctorientation may functionally provide nearly the same information asthree electrodes 302. In one embodiment a single stimulation electrode102 and an indifferent electrode (not shown) may be used to verify thata path for a surgical or other medical instrument is free of nerves 104.

FIG. 6A depicts one embodiment of an enlarged cross sectional view ofthe tip area 516 of stimulation electrode 302 a of FIG. 5. In certainembodiments the stimulation electrodes 302 are smaller than a dilator orother instrument typically used in a surgical procedure.

In one embodiment the stimulation electrode 302 is made of a 28 gaugestainless steel or other electrically conductive and medicallyappropriate wire 602. In one embodiment the wire 602 may be covered withTeflon or other insulating material 604 which is ground back to bevelboth the wire 602 and the insulating material 604. By beveling the wire602 and the insulating material 604 the tip 306 a is small while theshaft of the stimulation electrode 302 a remains large enough to besubstantially rigid. The small tip 306 a of the stimulation electrode302 a reduces the chance of hitting the nerve 104 directly, and reducesthe likelihood of injury to the nerve 104 if the stimulation electrode302 a touches the nerve 104.

FIG. 6B depicts one embodiment of an enlarged cross sectional view ofthe lead coupling area 518 of stimulation electrode 302 a of FIG. 5. Incertain embodiments the Teflon or other insulating material 604 isground back or otherwise removed from the wire 602 leaving a couplinglead 606 for coupling the stimulation electrode 302 a to the electricalsource 403 or control module 401.

Each electrode 302 is individually insulated with Teflon or anotherinsulating material 604 except for the tip 306 and the coupling lead606. While the embodiments illustrated in FIGS. 6A and 6B are discussedwith reference to stimulation electrode 302 a, one of skill in the artwill recognize that stimulation electrodes 302 b and 302 c may besubstantially similar to stimulation electrode 302 a.

FIG. 7 depicts one embodiment of a portion of a patient's spinal column702 with the electrode positioning module 410 positioning threestimulation electrodes 302 near the patient's spinal column 702. FIG. 7also illustrates one embodiment of the imaging module 414 having twoimaging devices 704.

In certain embodiments each imaging device 704 captures an image 706 ofa user's anatomy from a different angle. In one embodiment the images706 are captured from at least two angles. In certain embodiments theangles are offset by ninety degrees such that the images 706 show boththe x and y axis as well the z axis of a Cartesian coordinate system708. The images 706 may then be combined to give a three dimensionalimage of the user's anatomy to a physician or other operator. In theembodiment depicted in FIG. 7 the Cartesian coordinate system 708 isillustrated for clarity purposes only.

In certain embodiments the mapping module 408 uses distances calculatedby the distance module 406 to map the location of a nerve 104. In theembodiment illustrated in FIG. 7 several nerves 104 are depicted. Incertain embodiments the apparatus 400 may be used to map each of thenerves 104 in the manner described above.

The overlay module 418 overlays the map of the nerve(s) 104 on theimage(s) 706 captured by the imaging device(s) 704. In one embodimentthe stimulation electrodes 302 may be made of a material detectable bythe imaging module 414 such that the stimulation electrodes 302 areshown in the images 706 captured by the imaging devices 704. In anotherembodiment the marking module 420 may position a marker (not shown) madeof a material detectable by the imaging module 414 at or near theposition of each stimulation electrode 302. In certain embodiments theoverlay module 418 uses the stimulation electrodes 302 or the markers(not shown) as reference indicators to aid in positioning the map of thenerve 104 in an appropriate position on the images 706.

In certain embodiments a display unit 710 displays the images 706 of thepatient's anatomy along with the overlaid map of the nerve 104 so that aphysician or other user can avoid the nerve 104 in performing a medicalprocedure. In one embodiment each image 706 is displayed separately sothat the physician or other user can determine the position of nerve 104in three dimensions. In another embodiment the two images 706 may becombined and displayed on the display unit 710 using three dimensionalrendering as is known in the art.

FIG. 8 is a schematic block diagram depicting one embodiment of a method800 for mapping the location of a nerve 104 such as may be used with theapparatus 400 of FIG. 4. The method 800 starts 802 and a stimulationmodule 402 stimulates 804 a nerve with an electrical stimulation currentfrom at least one stimulation electrode 102. In certain embodiments thenerve 104 is stimulated 804 by three stimulation electrodes such asstimulation electrodes 302. In such an embodiment the nerve 104 may bestimulated 804 by each stimulation electrode 302 independently, that is,in certain embodiments a first stimulation electrode 302 a stimulates804 the nerve 104 first, a second stimulation electrode 302 b stimulates804 the nerve 104 second and a third stimulation electrode 302 cstimulates 804 the nerve 104 third.

In one embodiment each stimulation electrode 102 (or stimulationelectrodes 302) may stimulate 804 the nerve with a current that isincreased until a threshold muscle reaction is detected 806 by astimulation detection module 404. In certain embodiments the musclereaction results from the stimulation 804 of the nerve by the at leastone stimulation electrode 102 (or stimulation electrodes 302).

In certain embodiments the muscle reaction is detected 806 by aelectromyograph. In other embodiments the muscle reaction may bedetected 806 by physical observation of the patient's anatomy ormuscles.

A distance module 406 calculates 808 a distance between the at least onestimulation electrode 102 (or stimulation electrodes 302) and the nerve104 using current information from the at least one stimulationelectrode 102 (or stimulation electrodes 302) at a time of firstdetecting 806 the muscle reaction. In one embodiment the distance module406 uses Coulomb's law to calculate 808 the distance between the atleast one stimulation electrode 102 (or stimulation electrodes 302) andthe nerve 104. In other embodiments the distance module 406 uses anyformula that calculates 808 distance as function of a stimulationcurrent required to invoke an electromyogram response in the muscle.

In certain embodiments a mapping module 408 maps 810 a location on thenerve 104 using at least two distance calculated 808 and positioninformation of the at least one stimulation electrode 102 (orstimulation electrodes 302) for each of the at least two distancescalculated 808. In one embodiment the mapping module 408 uses at leastthree distances calculated 808 to map 810 the location on the nerve. Ineither case, each of the distances calculated indicates a sphericallocus of potential sites 108 (or spherical locus of potential sites 308)on the nerve 104 equidistant from a position of the at least onestimulation electrode 102 (or stimulation electrodes 302) when the atleast one stimulation electrode 102 (or stimulation electrodes 302)stimulates 804 the nerve 104.

In embodiments where distances are calculated 808 to map 810 a locationon the nerve 104, the location on the nerve 104 can be determined to liesomewhere on an intersection of the two of the spherical locus ofpotential sites which is a circle, such as the circle 214 created by theintersection of the two spherical locus of potential sites 208 asillustrated in FIG. 2 and discussed above. In certain embodimentspinpointing the location on a nerve 104 with an accuracy of circle 214may be enough to perform certain medical procedures. In one embodiment aphysician's or other professional's knowledge of a patient's anatomy maybe used in combination with the method presented herein to pinpoint alocation on the nerve 104 using only two distance calculations.

In other embodiments a medical procedure may call for greater accuracy.In such an embodiment three distances may calculated 808 to map 810 alocation on the nerve 104. Where three distances are used to map 810 thelocation on the nerve 104 the location can be determined to be at one oftwo positions 316 identified by the intersection of three sphericallocus of potential sites 308 as illustrated in FIG. 3 and discussedabove. In a further embodiment more than three distances may becalculated 808 to map 810 the location on the nerve 104 with greateraccuracy. Upon mapping 810 the location of the nerve, the method ends812.

FIG. 9 is a schematic block diagram depicting one embodiment of a method900 for mapping the location of a nerve 104. In certain embodiment themethod 900 begins 902 and the steps of method 800 are performed 904. Incertain embodiments the method 900 includes capturing 906 an image apatient's anatomy and overlaying 908 a map of the location on the nerve104 on the image of the patient's anatomy and the method ends 910. Inone embodiment the map of the location on the nerve 104 may be overlaid906 on the image of the patient's anatomy in three dimensions such thatthe physician may determine the location of the position on the nerve104 in three dimensions.

The present subject matter may be embodied in other specific formswithout departing from its spirit or essential characteristics. Thedescribed embodiments are to be considered in all respects only asillustrative and not restrictive. The scope of the subject matter is,therefore, indicated by the appended claims rather than by the foregoingdescription. All changes which come within the meaning and range ofequivalency of the claims are to be embraced within their scope.

What is claimed is: 1-20. (canceled)
 21. An apparatus configured tovisually display a location of a nerve in a patient, the apparatuscomprising: a control module configured to: determine a first locus ofpotential sites of the nerve equidistant from a first position on thepatient when a first stimulation electrode stimulates the nerve;determine a second locus of potential sites of the nerve equidistantfrom a second position on the patient when a second stimulationelectrode stimulates the nerve, wherein the second position is differentfrom the first position; and determine an intersection of the firstlocus of potential sites of the nerve and the second locus of potentialsites of the nerve to identify the location of the nerve; an imagingmodule configured to acquire an image of the patient's anatomy; and anoverlay module configured to visually overlay a map of the location ofthe nerve on the image of the patient's anatomy.
 22. The apparatus ofclaim 21, wherein the control module is configured to provide electricalcurrent from an electrical source to each of the first stimulationelectrode and the second stimulation electrode.
 23. The apparatus ofclaim 21, wherein the control module further comprises an electrodepositioning module configured to position each of the first stimulationelectrode and the second stimulation electrode.
 24. The apparatus ofclaim 21, wherein the first locus or second locus is spherical.
 25. Theapparatus of claim 21, wherein the first stimulation electrode and thesecond stimulation electrode are different.
 26. The apparatus of claim21, wherein the first stimulation electrode and the second stimulationelectrode are the same.
 27. The apparatus of claim 21, wherein thecontrol module is further configured to determine a third locus ofpotential sites of the nerve equidistant from a third position on thepatient when a third stimulation electrode stimulates the nerve anddetermine an intersection of the third locus of potential sites of thenerve with at least one of the first locus of the potential sites or thesecond locus of potential sites of the nerve to identify the location ofthe nerve.
 28. The apparatus of claim 27, wherein the first stimulationelectrode, the second stimulation electrode, and the third stimulationelectrode are different.
 29. The apparatus of claim 27, wherein thefirst stimulation electrode, the second stimulation electrode, and thethird stimulation electrode are the same.
 30. The apparatus of claim 27,wherein the third position is different from the first position and thesecond position.
 31. A method for visually displaying a location of anerve in a patient using an apparatus having a control module, animaging module, and an overlay module, the method comprising using thecontrol module, determining a first locus of potential sites of thenerve equidistant from a first position on the patient when a firststimulation electrode stimulates the nerve; using the control module,determine a second locus of potential sites of the nerve equidistantfrom a second position on the patient when a second stimulationelectrode stimulates the nerve, wherein the second position is differentfrom the first position; using the control module, determining anintersection of the first locus of potential sites of the nerve and thesecond locus of potential sites of the nerve to identify the location ofthe nerve; using the imaging module, acquire an image of the patient'sanatomy; and using the overlay module, visually overlay a map of thelocation of the nerve on the image of the patient's anatomy.
 32. Themethod of claim 31, further comprising, using the control module,providing electrical current from an electrical source to each of thefirst stimulation electrode and the second stimulation electrode. 33.The method of claim 31, further comprising, using the electrodepositioning module positioning each of the first stimulation electrodeand the second stimulation electrode.
 34. The method of claim 31,wherein the first locus or second locus is spherical.
 35. The method ofclaim 31, wherein the first stimulation electrode and the secondstimulation electrode are different.
 36. The method of claim 31, whereinthe first stimulation electrode and the second stimulation electrode arethe same.
 37. The method of claim 31, further comprising, using thecontrol module, determining a third locus of potential sites of thenerve equidistant from a third position on the patient when a thirdstimulation electrode stimulates the nerve.
 38. The method of claim 37,further comprising, using the control module, determining anintersection of the third locus of potential sites of the nerve with atleast one of the first locus of the potential sites or the second locusof potential sites of the nerve to identify the location of the nerve.39. The method of claim 38, wherein the first stimulation electrode, thesecond stimulation electrode, and the third stimulation electrode aredifferent.
 40. The method of claim 38, wherein the first stimulationelectrode, the second stimulation electrode, and the third stimulationelectrode are the same.